Essay: LIGO's Extended Family

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If you fly into Baton Rouge, and you have a window seat, you’ll notice a strange sight. The mosaic of pine forest near Livingston becomes oddly parted. A perfect L cuts through 8 km of foliage, with the long, skinny concrete arms of the Laser Interferometer Gravitational-wave Observatory visible in the clearing.

But if you fly over Hanford, Washington; Pisa, Italy; Hannover, Germany; or Tokyo, Japan, you’ll realize that giant L’s aren’t peculiar to Louisiana. The world actually has not one, but five large-scale gravitational-wave detectors. Each is devoted to the collective cause of detecting, for the first time, gravitational radiation streaming from neutron stars, black holes, and other giant, moving masses in space.

Gravity from the Ground

As landmark as LIGO is, its reach is limited. In fact, it can’t achieve its goals without a network of similar interferometers, instruments that use light waves to detect a gravitational wave’s minute warp on space. The network will be able to detect the same sources that LIGO does, thereby verifying its observations.

The other interferometers that share LIGO’s earthly burden — LIGO’s Hanford, Washington, facility, Italy’s VIRGO, Germany’s GEO, and TAMA in Japan — have so far undergone various “dress rehearsals” together to compare and contrast initial data. Real collective runs using all detectors will hopefully begin no later than 2006.

If three of the detectors notice the same gravitational signal at once, they will be able to pinpoint where the source lives in space, says Neil Cornish, a gravitational-wave researcher and astrophysicist at Montana State University. “Because gravitational waves travel at the speed of light, they’ll be slight delays in the arrival times at the different detectors around the world,” he explains. Researchers can easily calculate these differences to triangulate the source location in space.

Locating such sources is a key step in plotting a brand-new map of space using gravitational information. Researchers can then scan the spot thoroughly with conventional optical and radio telescopes to add even greater detail about these sources.

Gravity from Space

Plans are underway for another interferometer that will have a front-row seat to the cosmic objects that emit gravitational waves: a seat from space itself. Slated for launch in 2013, NASA’s Laser Interferometer Space Antenna (LISA) will boast the longest interferometer arms in the Universe--each 5 million km long!

As space provides a near-perfect vacuum, LISA doesn’t need steel tubes or concrete to encase its arms. Instead, a virtual L will be created using a trio of satellites that will orbit the Sun together in a fixed arrangement. Each satellite will have its own laser directed towards the other two. The travel time of the beams leaving the satellite will be compared to that of the beams arriving from the distant satellites. This will determine whether or not a gravitational wave has affected the “arm” length.

LISA’s superior location and keen instrumentation won’t render ground-based interferometers useless, however. The detectors will complement one another by capturing waves at different points along the gravitational spectrum. Ground-based instruments are tuned to pick up the spastic, infrequent, high-frequency gravitational waves like those shed by binary objects ready to collide after billions of years of spiraling toward one another. LISA’s instrumentation, on the other hand, will capture the much slower, longer-lasting, lower-frequency waves launched during the early stages of these binaries’ spiraling-in process, thousands of years before they collide. Strong gravitational signals from these binary sources will pile up LISA’s detector with “an embarrassment of riches,” predicts Cornish.

Because LISA will have little interference from outside noise, the low-frequency waves should show up as obvious spikes in the signal output. In addition, LISA should be able to pick up gravitational information from supermassive black holes, which radiate waves at frequencies ground-based interferometers can’t register. “If we haven’t detected a gravitational wave within a day or two of LISA starting operation,” says Cornish, “we’ll know that something’s gone horribly wrong.”

Probing Creation

As ambitious as LISA is, the search for gravitational waves won’t end there. Fast-forward 30 years into the future. At that time, Cornish anticipates being heavily involved in an even-more-next-generation interferometer that he describes as “LISA on steroids”: the Big Bang Observatory. Along with LISA, this observatory is a key component of the Beyond Einstein project, a collection of missions aiming to answer fundamental questions about the structure and evolution of the Universe. The space-based Big Bang Observatory could offer scientists an unprecedented view of the very earliest moments of our Universe’s expansionone seen through gravitational waves.

Gravitational waves first started escaping from the Big Bang at about 10 “35 seconds after it began 14 billion years ago. It’s taken that long for the gravitational information to reach our cosmic doorstep, and to this day, waves from this event continue to pass by. Because gravitational waves are affected little by intervening space objects, these ancient signals are theorized to be intact, yet faint. They’ve had quite a journey, after all.

“We expect these gravitational waves from the Big Bang to be fairly weak,” explains Cornish, “so we have to up the sensitivity of the instrument.” This translates into two supersized interferometers for the Big Bang Observatory. Each will resemble LISA, but they will be equipped with lasers over a thousand times more powerful and they will have much larger mirrors, each a few meters wide.

The behemoth instrument is very much in the conceptual stages. Cornish is currently determining how the endless stream of competing signals from binary sources will be subtracted from the observatory’s anticipated readings in order to get the purest possible Big Bang signal.

With five ground-based interferometers in place and two space projects in the works, the science community is putting quite a lot of stock into a theoretical signal that hasn’t even been officially detected yet. But Cornish, like most physicists, isn’t worried. “The most spectacular discovery for gravitational-wave observatories would be an absence of waves,” he says. “That would be the most revolutionary discovery of science this century.”